Three of the major color television systems at present in use are those known as the NTSC system, the PAL system and the PAL-M system. In each of these systems, each horizontal line of the TV signal comprises a negative-going sync pulse followed by the color burst, which is a sinusoidal signal. In the NTSC system, the phase of the color burst relative to horizontal sync is reversed on alternate lines, i.e., the phase changes by 180 degrees from line to line, whereas in the PAL system the phase of the color burst changes with respect to sync by a multiple of 90 degrees, plus 0.7 degrees per line. The additional 0.7 degrees corresponds to a frequency of 25 Hz. In PAL-M, the change is the same as in PAL, but omitting the additional 0.7 degrees. Thus, whereas in the NTSC system the sinusoidal form of the color burst can be seen when the signal is displayed on a waveform monitor at field rate, in the PAL system the 25 Hz offset results in a blur being seen in the burst interval, and in the PAL-M system multiple sine waves are seen, and therefore in both the latter systems it is necessary to view a selected line at a field rate in order to perceive the sinusoidal form of the burst.
In each of the three major systems, the color burst is said to be in phase with horizontal sync when the positive-going zero crossing of the extrapolated burst wave aligns with the 50 percent point of the leading edge of sync on line 1 of the composite signal. In the case of the NTSC system, this relationship applies to all odd-numbered lines owing to the reversal of phase of burst on alternate lines.
In a television studio, it is conventional to use a master sync generator to provide composite sync (horizontal and vertical sync information) and a continuous subcarrier wave at burst frequency to the cameras and other video signal sources in the studio. The cameras use the continuous subcarrier wave to generate the color burst. In order to ensure that the color burst is in phase with horizontal sync, the phase relationship between the sync information and the subcarrier wave is adjusted to bring the positive-going zero crossing of the subcarrier into alignment with the 50 percent point of the leading edge of sync. The phase difference between the continuous subcarrier wave and horizontal sync may be measured by overlaying subcarrier and sync on a dual trace oscilloscope and then adjusting the phase of sync to achieve the desired phase relationship. This, however, assumes that a dual trace oscilloscope is available for use, and this might not always be the case in a television studio. Moreover, the harmonics of the subcarrier tend to distort the sinusoidal form of the subcarrier wave. Further, the overlay technique is not usable with the PAL system owing to the progressive change in phase of subcarrier through the field. Conventional subcarrier to horizontal phase meters are not inherently accurate and are subject to drift, and therefore need to be calibrated repeatedly. Other devices purport to maintain correct phase relationship without providing a display, but certain of these devices are subject to inherent limitations, such as a need to set horizontal and vertical references and to use equalizing pulses.